A magnetic sensor utilizing a thin magnetic film traversed by an electric current I and across which a voltage drop can be detected is known and utilizes the magnetic resistance effect. According to this principle, when the thin magnetic film traversed by a current I is juxtaposed with a source of a magnetic field, the external magnetic field alters the electrical resistance of the film by the magnetic resistance effect and this resistance change, because of the passage of an electric current through the film can be manifested in a change in the voltage which is detected. The voltage thus represents the external magnetic field. The terminal through which the electric current is applied can be considered means for connecting an electric current source to the film while the terminals across which the voltage is detected can be considered means connected across the film for measuring the potential difference thereacross.
The sensors of this type can be used for reading magnetically stored data, for example solid plate magnetic storage and so-called hard disks, floppy disks, magnetic tape and other storage sources. They may be used for any other purpose in which high resolution in the detection of local magnetic fields is necessary and desirable.
In such applications, the magnetic fields emanating from the data storage medium are converted into changes in the magnetization direction or domain structure in the magnetic layers of the magnetic field sensor. In detectors used heretofore, because of the so-called anisotropic magnetic resistance effect, there is a change in the electrical resistance and thus a potential difference U which is a detectable electrical signal. The thus obtained electrical signal can be processed by conventional circuitry.
With certain films, for example Permalloy, the change in magnetic resistance upon which the measurement signal depends, is a maximum of about 3%. The signal level is thus small and since the value of the signal depends upon the signal/noise ratio, it is highly desirable to maximize the output signal U.